Vascular adhesion protein-1 (VAP-1) is a semicarbazide-sensitive amine oxidase (SSAO), which is abundantly present in human plasma. VAP-1 is an ectoenzyme comprising a short cytoplasmic tail, a single transmembrane domain, and an extracellular domain with large and high glycosylation containing the center of activity. In addition, VAP-1 exists not only as a membrane-bound form in the endothelium, but also as a soluble form in serums (soluble VAP-1, sVAP-1). This form was shown to be a product cleaved from the membrane-bound VAP-1, and appears to have similar properties as the tissue-bound form. It has been also reported that VAP-1 is normally stored in intracellular granules within endothelial cells, but when an inflammatory response is evoked in response to inflammatory stimuli, it is translocated onto the cell membrane, and its expression is unregulated, and therefore, it is expressed more strongly in inflamed tissues than in normal tissues.
Substrates for VAP-1 include endogenous methylamine and aminoacetone as well as some xenobiotic amines such as tyramine and benzylamine.
VAP-1 has two physiological functions: the first is amine oxidase activity stated earlier in this section, and the second is cell adhesion activity. Due to these two activities, VAP-1 has been shown to play a key role in the leakage of inflammatory cells as it acts as an adhesion protein for leukocytes in inflamed sites [Trends Immunol. (2001) 22: 211]. VAP-1-deficient transgenic mice are healthy, develop normally, and fertile, and phenotypically normal, but exhibit a marked decrease in the inflammatory responses evoked in response to various inflammatory stimuli [Immunity. (2005) 22: 105].
In addition, inhibitory activity of VAP-1 in multiple animal models of human diseases (e.g., carrageenan-induced paw inflammation, oxazolone-induced colitis, lipopolysaccharide-induced lung inflammation, collagen-induced arthritis, endotoxin-induced uveitis) by the use of antibodies or small molecules has been shown to prevent leukocyte from rolling, adhering, and leaking, and reduce levels of inflammatory cytokines and chemokines, thereby reducing the severity of the disease [Eur J Immunol. (2005) 35: 3119; J Pharmacol Exp Ther. (2005) 315: 553; Annu Rep Med Chem. (2007) 42: 229; FASEB J. (2008) 22: 1094]. Inflammation is the first reaction of the immune system to infection or stimulus and in such a process, the movement of leukocytes into the tissue through circulation is an important step. The leukocytes are first bound to adhesion proteins and then adhered to the endothelium before they start to pass through blood vessel walls. VAP-1 is highly expressed in endothelial venules (HEV) such as high endothelial venules in lymphoid organs, as well as hepatic sinusoidal endothelial cells, (HSEC), smooth muscle cells, and adipocytes. The VAP-1 expression on the cell surface of endothelial cells is strictly regulated and is increased during inflammation. VAP-1 activates NF-κB when it is present in the substrate, and the NF-κB is activated within the HSEC while E-selectin and chemokine IL-8 that are other adhesion molecules are upregulated ex vivo. This suggests that VAP-1 may be a key factor for the regulation of the inflammatory response, and it seems therefore likely that VAP-1 inhibitors may be effective anti-inflammatory drugs in a wide range of human diseases.
Nonalcoholic fatty liver disease (NAFLD), histologically, encompasses simple steatosis, nonalcoholic hepatosteatosis (NASH), and liver cirrhosis. Among these, unlike simple steatosis (non-alcoholic fatty liver, NAFL), NASH potentially progresses to liver cirrhosis and hepatoma (hepatocellular carcinoma). In NASH, insulin resistance is known to play an important role in the progression of disease, along with oxidative stress, inflammatory cascade, and fibrosis. In patients with NAFLD, sVAP-1 levels were found to be elevated, and in VAP-1 knockout (K/O) mice, carbon tetrachloride-induced liver fibrosis was reduced compared with that in wild type animals. In addition, improvement of liver fibrosis by VAP-1 inhibition following administration of VAP-1 antibody was identified by histological changes [J Clin Invest (2015) 125: 501]. Thus, VAP-1 was found to be associated with NASH in clinical studies and animal models of diseases. Inhibitory activity of VAP-1 in the carbon tetrachloride-induced animal model appears to be due to a reduction in infiltration of leukocytes such as T cells, B cells, NKT cells, and NK cells observed in liver fibrosis, and VAP-1 inhibitors have the potential for treating fibrotic diseases.
Thus, a substance that inhibits VAP-1 may be applied to prevention and treatment of various inflammatory diseases and fibrotic diseases.